An A/D conversion apparatus has been proposed that generates most significant bits of an A/D conversion result by performing A/D conversion of an analog input signal using a delta-sigma (ΔΣ) modulator, generates least significant bits by performing a cyclic A/D conversion of the residue of the quantization, and outputs an A/D conversion result of the analog input signal by combining the most significant bits and the least significant bits.
Each of a delta-sigma modulator and a cyclic A/D converter has an operational amplifier as their main part. In a technique disclosed in JP-B2-04862943 corresponding to U.S. Pat. No. 7,289,054 B2, a delta-sigma modulator and a cyclic A/D converter share an operation amplifier and are operated by operation mode switching.
In order to use an operational amplifier as a delta-sigma modulator and a cyclic A/D converter, multiple switches are required to change a connection condition between the operational amplifier and a capacitor which works in conjunction with the operational amplifier to form an integrator circuit or an amplifier circuit.
A switch serves as a resistor equivalently when it is ON. For this reason, if a switch is connected between an input of an operational amplifier and a capacitor, an operation speed of a circuit constructed with the operational amplifier may be degraded. The degradation in the operation speed due to the switch may be reduced by reducing an ON-resistance of the switch. However, the switch needs to be increased in size to reduce its ON-resistance. Accordingly, its circuit area is increased. In particular, since a parasitic capacitance of a switch to be connected to the operational amplifier increases with an increase in the circuit area, a feedback factor of a feedback loop constructed with the operational amplifier, the switch, and the capacitance may be reduced. Accordingly, the operation speed and accuracy of processing the analog input signal may be reduced.
Further, since a gain (generally, two or four times) of a closed loop which is formed when the operational amplifier is used as a cyclic A/D converter is greater than a gain (generally, one or less times) of a closed loop which is formed when the operational amplifier is used as a delta-sigma modulator, a gain-bandwidth product which a circuit constructed with the operational amplifier is required to have is larger. In particular, it is likely that an operation speed of the cyclic A/D converter becomes a bottleneck of an operating frequency of the circuit. For this reason, the switch exerts a larger influence on the whole operation speed when the operation amplifier is used as a cyclic A/D converter than when the operation amplifier is used as a delta-sigma modulator.